Figure 2. Relation between ozone and the ratio of erythemally weighted UV irradiance over total global irradiance for various solar elevations. From [2].

This relation can be verified with measurements under clear sky conditions at fixed SZA [2]. Measurements at the High Alpine mountain station Jungfraujoch (3576 m above sea level, Switzerland) are only marginally influenced by aerosols, and the effect of varying albedo is minimized by analysing the ratio Ger/G. This shows clearly the relation following the power law, when in a log-log plot the relation becomes linear with the RAF as the slope of the linear regression line (Fig. 2). These measurements give a value for the RAF of 1.1, which is in very good agreement with model calculations using radiative transfer models. This means that a reduction of ozone by 1% will result in an increase of erythemally weighted UV irradiance by 1.1%.

1.3. Effect of Aerosols

Aerosols in the atmosphere can scatter or absorb UV radiation, depending on their chemical composition. In most cases the absorbing component is of the order of 1-10% of the whole extinction (absorption plus scattering), only when aerosols originate from great fires the absorbing component might be higher. As a consequence of the scattering component of aerosols the diffuse irradiance is increased and the direct irradiance is decreased. The quantitative amount of these effects depends on the amount and type of aerosols, on their size distribution and on the vertical distribution in the atmosphere. Aerosol extinction depends only slightly on wavelength with higher values at shorter wavelengths. As an example for the variable attenuation effect of aerosols on erythemally weighted irradiance in Fig. 3 the attenuation relative to an aerosol free situation is shown for measurements at a campaign near Athens, Greece, in summer 1996 [3]. For many days the attenuation is in the order of 5 to 15%, but on some days (depending on the local wind direction) the attenuation can reach values up to 30 and 35%, relative to an aerosol free atmosphere. It is interesting to mention that during the 20 days of this campaign the variability of aerosols had a greater effect on erythemally weighted irradiance than the variability of ozone.

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